1. Field of the Invention
The present invention relates to a vent plug for a tire vulcanization mold mounted in a vent hole of the tire vulcanization mold, a tire vulcanization mold and a method for manufacturing a pneumatic tire by using the tire vulcanization mold.
2. Description of the Related Art
Conventionally, air is trapped between an inner surface of a tire vulcanization mold and an unvulcanized tire during vulcanization molding. In order to ventilate the air out of the mold, a tire vulcanization mold used in a vulcanization process of a tire is formed with many narrow exhaust holes called a vent hole. When a tire is subjected to vulcanization molding process, an unvulcanized tire is heated within a mold heated to a high temperature. During this process, the unvulcanized tire is expanded and a viscosity of a rubber surface decreases. Therefore, rubber flows into the vent holes during vulcanization molding of the tire and projections, so-called spews, which extend like a beard on a tire surface, are formed. Therefore, after completing the vulcanization molding process of the tire, a trimming process to remove the spews is indispensable in post processing. Accordingly, there is such a problem that works efficiency and productivity is reduced.
To solve this problem, in order to improve the work efficiency and the productivity by eliminating the trimming process after the vulcanization molding process of the tire, there have been proposed the following tire vulcanization molds, in which a vent hole is arranged to close while releasing the air to thereby prevent rubber from flowing into the vent hole.
Japanese Unexamined Patent Publication No. 10-296735 discloses a vent hole open/close valve for a mold, which includes a vent base having a vent hole formed therein, an open/close valve body movable with respect to the vent base on opening/closing the vent hole, a spring unit formed integrally with the vent base to open the vent hole, and a valve seat formed between the vent base and the open/close valve body. The above vent hole open/close valve is arranged to be closed by rubber within the mold when the rubber comes into contact with an upper end face of the open/close valve body and pushes the same. This arrangement prevents the rubber from flowing into the vent hole after the air is ventilated out of the mold. As a result, spews are prevented from being generated on the surface of the tire manufactured using the mold; and thus, the trimming process after the vulcanization molding of the tire is eliminated.
However, the above vent hole open/close valve has the following problems. That is, the spring unit needs to be formed integrally with the vent base and a complicated open/close mechanism is provided thereto. Therefore, a price of the valve is considerably high and the valve is difficult to manufacture. Further, the valve is used in the open/close valve body combined with a spring unit fitted thereto. Therefore, the spring unit may often drop off from the open/close valve body; and thus the valve is poor in durability.
Japanese Unexamined Patent Publication No. 10-146840 discloses a vent hole open/close valve for a mold, which includes an open/close valve body and a spring unit attached integrally to the open/close valve body to open the open/close valve body. The vent hole open/close valve is arranged so that the open/close valve body opens/closes the vent hole by using or against a spring force of the spring unit to prevent the rubber from flowing into the vent hole after releasing the air out of the mold. As a result, spews are prevented from being generated on the tire surface manufactured by using the mold; and thus the trimming process after the tire molding process can be eliminated.
However, in the above vent hole open/close valve, the open/close valve body is repeatedly opened/closed by the spring unit. Due to a load applied to the spring unit, the durability of the spring unit decreases as time passes.
Japanese Unexamined Patent Publication No. 2005-28589 discloses such a plug used for a vent hole of a tire vulcanization mold that a plug body includes a cylinder portion fitted to vent cylinder of a vent hole and a head portion that comes into contact with a recess portion having a step. The cylinder portion is formed with a communication path communicated with the vent cylinder, and the head portion is provided with an open/close valve for allowing the communication path and the inside of the mold to be communicated with each other or to be closed therebetween. In the plug described above, the open/close valve has a function as a valve to open the vent hole before placing a tire into the mold; and after the tire is placed, to close the vent hole utilizing a pressure given by the tire.
However, in the above plug, due to a structure thereof, the open/close valve cannot completely close the vent hole utilizing the pressure given by the tire. Rubber may flow into the vent hole through the communication path. This problem noticeably appears when the plug is repeatedly used. As a result, spews are generated on the surface of the tire manufactured using the mold for forming the tire provided with the above-described plug; and thus, the trimming process is required after molding the tire. Further, when the plug is made of a heat-resistant resin, in particular when the plug is repeatedly used, the plug tends to deteriorate quickly resulting in a reduction of the durability of the plug.